Motor control device

ABSTRACT

A motor control device includes a bridge circuit having semiconductor switching elements disposed in each arm thereof and being adapted for the connection of a motor between pairs of arms. The semiconductor switching elements are selectively rendered conductive to alternately drive the motor in forward and reverse directions, and certain ones of a semiconductor switching elements may be simultaneously rendered conductive to form a closed loop which short-circuits the motor and causes dynamic braking. The motor control device is capable of instantaneously stopping the rotation of the motor and is capable of rapidly reversing the motor&#39;s direction of rotation. The device is particularly useful in an automatic focusing apparatus or an automatic diaphragm setting apparatus in a camera.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to motor control devices forcontrolling the direction of rotation and the braking of a DC motor, andmore particularly to devices useful for controlling a motor employed inan automatic focusing apparatus or an automatic diaphragm settingapparatus in a camera.

2. Description of the Prior Art

In various apparatus employing a DC motor (hereinafter referred tosimply as a motor) driven from a DC power source, it is generallyrequired not only to control the rotation of the motor in one direction,but also to stop the motor instantaneously and to reverse its directionof rotation. The ability to instantaneously stop a motor and quicklychange its rotational direction, is essential when the motor is employedin a servo system, where rapid and precise control are generallyimportant.

Within limits, a servo system having some of the above-mentionedcharacteristics may be obtained by increasing the sensitivity or theresponsiveness of the servo system. However, increased sensitivityenhances the hunting phenomenon caused by the inertia of the motor andthe driven members, and results in reduced stability in operation. Manyattempts have been made to lessen the hunting phenomenon. One approachis to increase the reduction ratio of a gear train which is driven bythe shaft of the motor. Another known approach is to broaden the deadrange of the servo system. However, all the known approaches eitherunduly limit the sensitivity of the servo system and/or reduce theprecision with which the system can be controlled.

Moreover, even for non-servo systems or apparatus which simply use amotor as a driving source, where the inertia of driven members, such astorque transmission members, for example, engaged by the shaft of themotor is large, it is very difficult to stop the rotation of the motorinstantaneously or to exercise precise control over its rotation.

Thus, all of the apparatus requiring rapid and precise control of amotor, particularly those apparatus employing a miniature DC motor, suchas in a servo mechanism for an automatic diaphragm setting apparatus oran automatic focusing apparatus in a camera, always involve theabove-mentioned problems.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the invention to provide a motor controldevice which can permit the realization of a servo system having highsensitivity, i.e., high response speed, a narrow dead range and highprecision.

Another object of the invention is to provide a motor control devicewhich is capable of instantaneously stopping a motor, and which iscapable of rapidly reversing the motor's direction of rotation.

Briefly stated, a motor control device in accordance with the inventionmay comprise a bridge circuit having switching elements disposed in eachof the arms of said bridge circuit, the bridge circuit adapted forconnection of a motor between pairs of arms, first means for selectivelyrendering certain ones of the switching elements conductive toalternately drive the motor in forward and reverse directions, andsecond means for rendering certain ones of the switching elementsconductive to form a closed loop which short-circuits the motor andcauses dynamic braking of the motor.

Other objects, features, and advantages of the invention will becomemore fully apparent from the following description and the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing a first embodiment of a motorcontrol device in accordance with the invention;

FIG. 2 is a schematic diagram of a control circuit useful forcontrolling the motor control device of FIG. 1;

FIGS. 3A-3J are timing diagrams showing the relative phases of varioussignals in the circuit of FIG. 2;

FIG. 4 is a schematic diagram of another control circuit useful forcontrolling the motor control device of FIG. 1; and

FIG. 5 is a circuit diagram showing a second embodiment of a motorcontrol device in accordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a first embodiment of a motor control device inaccordance with the invention. As shown, a first pair of seriesconnected semiconductor switching elements 1 and 4 and a second pair ofseries connected semiconductor switching elements 2 and 3 are connectedin parallel to a DC power source 6. Switching elements 1-4, which arepreferably transistors, constitute a bridge circuit, one transistorbeing disposed in each arm of the bridge circuit. A motor 5 is connectedbetween the neutral points of the bridge circuit, being connected to apoint between transistors 1 and 4 of the first pair and to a pointbetween transistors 2 and 3 of the second pair.

Transistors 1 and 2 lie in a first pair of opposing diagonal arms of thebridge circuit, and transistors 3 and 4 lie in a second pair of opposingdiagonal arms of the bridge circuit. (As used herein, the term "opposingdiagonal arms" refers to arms of a bridge circuit which lie parallel toone another on opposite sides of the bridge circuit when the bridgecircuit is laid out in a conventional diamond-shaped configuration. Asalso used herein, two arms of the bridge circuit are "adjacent" whenthey are connected together at one of their ends to one side of avoltage source applied to the bridge circuit and their opposite ends areconnected to different neutral points of the bridge. Accordingly,transistors 1 and 3 are in adjacent arms, and transistors 2 and 4 are inadjacent arms.) As shown in FIG. 1, transistors 1 and 4 and transistors2 and 3 are preferably of different semiconductor types, transistors 1and 3 being PNP transistors and transistors 2 and 4 being NPNtransistors. Transistors 1 and 4 comprise a first set of complementaryconnected PNP and NPN transistors, and transistors 2 and 3 comprise asecond such set. Accordingly, transistors 1-4 are all operated in agrounded emitter configuration with the motor connected to theircollectors as a load.

As will be explained hereinafter, when transistors 1 and 2 are turned onand they conduct, current flows through motor 5 in the direction of thearrow in FIG. 1, causing the motor to turn in a first direction, e.g.,the forward direction. Conversely, when transistors 3 and 4 conduct,current flows through motor 5 in the opposite direction to the arrow,causing the motor to turn in the opposite direction, e.g., the reversedirection. Moreover, if the motor is rotating in either direction andthe transistors in adjacent arms of the bridge circuit are turned on,e.g., transistors 2 and 4 or transistors 1 and 3, a closed loop will beformed, short-circuiting the motor. The back electromotive force (emf)generated by the motor due to its rotation will be absorbed in theclosed loop and cause dynamic braking of the motor, instantaneouslystopping its rotation. Accordingly, by selectively controllingtransistors 1-4, the direction of rotation of the motor can becontrolled and its rotation can be halted as desired.

To control transistors 1-4, transistor drivers 7, 8, and 9 may beemployed, as illustrated in FIG. 1. Transistor 7 has its emitterconnected to the base (control terminal) of transistor 1 and itscollector connected to the base of transistor 2 through a resistor 14.Similarly, the emitter of transistor 8 is connected to the base oftransistor 3 and its collector is connected to the base of transistor 4through a resistor 15. The base of transistor 7 may be connected througha resistor 17 to a terminal 21, and the base of transistor 8 may beconnected through a resistor 18 to a second terminal 22. Transistor 9,which is used for dynamic braking, has its emitter connected to thepositive terminal of power source 6 and its collector connected throughdiodes 10 and 11 and resistors 14 and 15 to the bases of transistors 2and 4, respectively. The base of transistor 9 may be connected through aresistor 16 to a third terminal 20.

When terminals 20, 21 and 22 are all open (or have a "high" signal levelapplied to them corresponding to the positive potential of power source6) all of the transistors in FIG. 1 are off (non-conducting). If a "low"signal (corresponding to the negative potential of power source 6) isapplied to terminal 21, transistors 1 and 7 both will be renderedconductive. The collector current of transistor 7 will flow throughresistor 14 into the base of transistor 2, rendering transistor 2conductive also. This allows the current to flow through motor 5 in thedirection of the arrow in FIG. 1 so that the motor will be driven intoforward rotation. Similarly, if a low signal is applied to terminal 22,transistors 3 and 8 will become conductive and, in turn, transistor 4will be rendered conductive. Accordingly, the current in motor 5 willflow in the opposite direction to the arrow in FIG. 1 so that the motorwill be driven in the reverse direction.

To brake the motor, both terminals 21 and 22 are first opened (orsupplied with a high signal) so that transistors 7 and 8 are both turnedoff. Thereafter, when a low signal is applied to terminal 20, transistor9 will be turned on, and its collector current will flow into the basesof transistors 2 and 4 through diodes 10 and 11 and resistors 14 and 15.Accordingly, transistors 2 and 4 will be simultaneously turned onforming a closed loop which short-circuits the motor and absorbs theback electromotive force (emf), causing the motor to be instantaneouslystopped. Upon closing of the loop, one of transistors 2 or 4 will beoperated in a reverse direction, depending upon the polarity of the backemf. For example, if the back emf generated by the motor is positive atterminal 26 of the motor and negative at terminal 28, transistor 4 willhave a collector current flow in the forward direction, i.e., from itscollector to its emitter. However, in transistor 2, which is a NPN type,the current will flow in the reverse direction, i.e., from its emitterto its collector. In this manner, the back electromotive force will beabsorbed, causing dynamic (or rheostatic) braking of the motor. When thepolarity of the back emf is reversed with respect to the above example,NPN transistor 4 will have a current flow in the reverse direction.

As is apparent from the foregoing, the control signals applied tocontrol terminals 20, 21 and 22 should be independent, and their timingshould be controlled so that a control signal is applied to only oneterminal at a time. If control signals are applied to two or moreterminals at the same time, power source 6 will be short circuited.

FIG. 2 illustrates a control circuit useful for generating controlsignals having the proper timing for controlling the device of FIG. 1.The control circuit of FIG. 2 is particularly adaptable for use in anautomatic focusing apparatus in a camera, in which an objective lens orthe like is moved along the optical axis for focusing by an amountcorresponding to a measured distance value between the camera and theobject to be photographed. Motor 5 may be used to drive the objectivelens.

As shown in FIG. 2, the control circuit has input terminals 30 and 31,which are adapted to receive sinusoidal signals from a well-known typeof focus detecting mechanism (not illustrated). The relative phase shiftbetween the signal applied to input terminal 30 and the signal appliedto input terminal 31 is proportional to the difference between the imageplane formed by the objective lens for a particular focus position ofthe lens and the film plane of the camera. For example, when the phaseof the signal applied to input terminal 30 is advanced with respect tothe phase of the signal on input terminal 31, the camera is out-of-focusin a first direction, e.g., focused in front of the subject, whereaswhen the phase shift is reversed, the camera is out-of-focus in theopposite direction, e.g., focused to the rear of the subject. When thephases of the two signals coincide or approximately coincide, the camerais in-focus. From the signals on input terminals 30 and 31, the controlcircuit of FIG. 2 generates the appropriate control signals on terminals20-22, which are connected to correspondingly numbered terminals in thecircuit of FIG. 1, to drive the lens to an in-focus condition, as willnow be described.

The input sinusoidal signals on terminals 30 and 31 are introduced intocomparators CP1 and CP2, which compare the input signals to a referencevoltage (V_(ref)) and shape them into rectangular signals having thesame relative phase shift. The output signals from comparators CP1 andCP2 are applied to the D input and the clock (CK) input, respectively,of a D flip-flop 35, which latches to the value of the D input when theinput of CK goes high. The output of flip-flop 35 is indicative of thephase advance or delay between the input signals. The output signal fromeach comparator is also input into both an OR gate G1 and an AND gateG2. The outputs of OR gate G1 and AND gate G2 correspond to the amountof phase shift between the output signals from the comparators. Theoutput of OR gate G1 is applied to a one shot multivibrator 38 whichoutputs a pulse having a predetermined time duration each time theoutput of G1 goes high, being triggered on the leading edge of the pulseoutput from G1. The output pulse from multivibrator 38 is introducedinto the D input of a second D flip-flop 39, and the output of AND gateG2 is introduced into the clock (CK) input of flip-flop 39. When theoutput of G2 goes high, flip-flop 39 latches to the value of the outputsignal from multivibrator 38. The Q and Q outputs of flip-flop 35 areintroduced into NAND gates G3 and G4, respectively, along with the Qoutput of flip-flop 39. The outputs from gates G3 and G4 constitute thecontrol signals on terminals 22 and 21, respectively, for driving themotor in the reverse and the forward directions, respectively. Thebraking control signal on terminal 20 is derived from a second one shotmultivibrator 40 connected to the Q output of flip-flop 39.

As previously mentioned, based upon the relative phase between the inputsignals on terminals 30 and 31, the control circuit of FIG. 2 generatesthe appropriate control signals on terminals 21 and 22 to drive themotor in either the forward direction or the reverse direction,depending upon the out-of-focus condition of the camera. When anin-focus condition is obtained, the output of multivibrator 40 goes lowto cause braking of the motor.

The operation of the control circuit of FIG. 2 may be best understood byreference to the timing diagram of FIG. 3. The outputs of comparatorsCP1 and CP2 are represented by FIGS. 3A and 3B, respectively. Assumingthat prior to time T1 the camera is out-of-focus toward the rear, andthat at time T1 the camera is in-focus, FIGS. 3A and 3B show that theoutput signal of CP1 is phase delayed relative to the output signal ofCP2 prior to time T1. At time T1 when in an in-focus condition isobtained, the output signals of CP1 and CP2 are in phase. FIGS. 3C and3D represent the outputs of gates G1 and G2, respectively. FIG. 3Erepresents the output of multivibrator 38. As shown, each time theoutput of gate G1 goes high, multivibrator 38 issues an output pulsehaving a predetermined time duration which is small with respect to theoutput signals of CP1 and CP2.

Prior to time T1, the Q output of flip-flop 39 is low, as shown in FIG.3F, and its Q output is high. On the other hand, prior to time T1, the Qoutput of flip-flop 35 is low and its Q output is high, as shown inFIGS. 3G and 3H, respectively. Since NAND gate G3 receives a high inputfrom flip-flop 39 and a low input from flip-flop 35, its output is high.However, the signals applied to the two inputs of NAND gate G4 are bothhigh, so that the output of G4 is low, as shown in FIG. 3J. Therefore,transistor 8 in FIG. 1 is turned off and transistor 7 is conductive,resulting in a current flow through the motor in the direction indicatedby the arrow of FIG. 1. This moves the objective lens toward thein-focus position.

At time T1 when an in-focus condition is obtained, the output ofmultivibrator 40 goes low for a predetermined time duration t (FIG. 3I).At the same time, the outputs from gates G3 and G4 both go high.Therefore, transistors 7 and 8 are both turned off, and transistor 9 isturned on for a time equal to t. This renders transistors 2 and 4conductive and, as previously described, instantaneously stops motor 5by dynamically braking the motor. Time t may be selected to have aduration which is sufficient to insure that motor 5 is completelystopped.

FIG. 4 illustrates another embodiment of a control circuit which,similar to the control circuit of FIG. 2, outputs control signals onterminals 20-22 to control driver transistors 7-9 of FIG. 1. The controlcircuit of FIG. 4 is designed to accept on input terminal 50 a voltagelevel from the output of a servo error amplifier or the like, and isparticularly adaptable for an automatic diaphragm setting apparatus,another type of automatic focusing apparatus whose output is a voltagelevel corresponding to the focus condition of a camera, and other motordriven systems.

In FIG. 4, a reference voltage V is divided by three resistors 51, 52,and 53 to form two reference voltages on terminals 54 and 55 which areapplied to the non-inverting inputs of comparators CP3 and CP4,respectively. The resistors are preferably selected so that thedifference between the two reference voltages is relatively small. Theinput voltage on terminal 50 is applied to the inverting input of thecomparators as a common signal voltage. The outputs of comparators CP3and CP4 are introduced through inverting amplifiers 57 and 58,respectively, into a NAND gate G7. The output of G7 is the controlvoltage on terminal 22 for driving the motor in the reverse direction.The outputs of the comparators are also introduced into a second NANDgate G8, the output of which is the control voltage on terminal 21 fordriving the motor in a forward direction. The output of comparator CP3and the output of comparator CP4 inverted by amplifier 58 are alsointroduced into AND gate G9, the output of which is used to trigger aone shot multivibrator 56. The output of multivibrator 56 is the brakingcontrol signal on terminal 20.

When the input signal voltage on terminal 50 is coincident with themedial value between the two reference voltages on terminals 54 and 55,the output from comparator CP3 is high and the output from comparatorCP4 is low. Accordingly, the output signals from gates G7, G8, and G9are all high, and multivibrator 56 outputs a low signal on terminal 20for a predetermined time duration. This turns both transistors 7 and 8(FIG. 1) off and turns transistor 9 on for the duration of the outputsignal from multivibrator 56, causing dynamic braking of the motor.

When the signal voltage on terminal 50 is higher than the referencevoltage on terminal 54, the outputs of comparators CP3 and CP4 are bothlow, and, therefore, the outputs of gates G7 and G9 are low, whereas theoutput of gate G8 is high. This turns transistor 7 off and renderstransistor 8 conductive so that the motor is driven in a reversedirection. Conversely, if the signal voltage on terminal 50 is lowerthan the reference voltage on terminal 55, the outputs from CP3 and CP4are both high. Accordingly, the output of gate G7 is high and theoutputs of gates G8 and G9 are low, rendering transistor 7 conductiveand turning off transistor 8, so that the motor is driven in a forwarddirection.

FIG. 5 illustrates another embodiment of a motor control circuit inwhich transistors 1-4 may be driven directly by a digital logic controlcircuit, and driver transistors 7-9 (and their associated circuitry) areeliminated. In FIG. 5, motor 5 and transistors 1-4, which form thebridge circuit, may be the same as those in the first embodimentillustrated in FIG. 1. The circuit of FIG. 5 has only two signal inputterminals 70 and 71. Input terminal 70 is used to control the directionof rotation of the motor depending upon whether the input signal onterminal 70 is high or low. Input terminal 71 is used for dynamicallybraking the motor whenever its input signal is high. The circuit of FIG.5 is formed to operate in a so-called stop priority mode. Thus, dynamicbraking is always effected on the motor when the input signal onterminal 71 is high, irrespective of the level of the signal then beingapplied to input terminal 70.

As shown, an input signal on terminal 70 is applied directly to OR gatesG14 and G17, and is applied through an inverter amplifier 72 to OR gatesG15 and G16. The outputs of gates G14 and G15 are connected throughresistors 74 and 75, respectively, directly to transistors 3 and 1,respectively. The outputs of gates G16 and G17 are introduced into ANDgates G18 and G19, respectively, through resistors 76 and 77. The inputsignal on terminal 71 is applied as a second input to gates G14-G17, andto the inverting input of an inverting OR gate G20, the output of whichis connected to gates G18 and G19. The input signal on terminal 71 isalso applied to a one shot multivibrator 73, which outputs a pulsehaving a predetermined time duration to gate G20.

The circuit of FIG. 5 operates in the following manner. Assuming thatthe input signal on terminal 71 is low, i.e., braking is not desired,when a high level signal is applied to input terminal 70, the outputs ofOR gates G14 and G17 will go high, and the outputs of OR gates G15 andG16 will be low. Since the low level signal on input terminal 71 isapplied directly to one input of gate G20, the output from gate G20 willgo high, causing AND gate G19 to go high. This renders transistors 1 and2 conductive so that current will flow through motor 5 in the directionindicated by the arrow in FIG. 5 (forward direction). Conversely, if alow level signal is applied to input terminal 70, the outputs of gatesG14 and G17 will be low and the outputs of gates G15 and G16 will gohigh. Accordingly, transistors 3 and 4 will be rendered conductive sothat the current in motor 5 will flow in the opposite direction to thearrow in FIG. 5, causing the motor to rotate in the reverse direction.

Irrespective of the level of the signal on terminal 70, when a highlevel signal is applied to terminal 71, the outputs of gates G14-G17will all go high. When the high level signal is applied to terminal 71,multivibrator 73 will output a high level signal having a predeterminedtime duration. As a result, the output of gate G20 will go high for atime equal to the time duration of the output from multivibrator 73.This will open gates G18 and G19, rendering transistors 2 and 4conductive and causing dynamic braking of the motor. Since the outputsfrom gates G14 and G15 are high, transistors 1 and 3 will be cut off.When the output of multivibrator 73 goes low, (after the predeterminedtime duration) the output of gate G20 will also go low, closing gatesG18 and G19 and turning transistors 2 and 4 off.

The arrangement of the circuit of FIG. 5 permits the application of astop signal to brake the motor as desired, irrespective of the presenceof a signal on terminal 70 for driving the motor in a particulardirection. Therefore, the arrangement permits the realization of arather simplified circuit for controlling the direction of rotation ofthe motor. The input signals on terminals 70 and 71 of the circuit ofFIG. 5 may be derived from control circuits such as illustrated in FIGS.2 and 4, for example.

In the illustrated embodiments, the transistors 1-4 which constitute thebridge circuit have been disclosed as a combination of PNP and NPNtransistors. However, the bridge circuit may also be formed using onlyNPN transistors or PNP transistors. Also, to handle motors having largewattage ratings, the transistors constituting the arms of the bridgecircuit may comprise parallel or Darlington connected transistors.Although in the illustrated embodiments, transistors 2 and 4 have beenutilized to form the closed loop which short-circuits the motor fordynamic braking, it should be understood that transistors 1 and 3 alsomay be used for this purpose instead of transistors 2 and 4. From theforegoing, it is apparent that a motor control device in accordance withthe invention provides a relatively simple and effective means forcontrolling a motor. The motor can be instantaneously stopped and itsdirection of rotation reversed, permitting the realization of a servosystem having high sensitivity, high response and high precision.

While preferred embodiments of the invention have been shown anddescribed, it will be apparent to those skilled in the art that changescan be made in these embodiments without departing from the principlesand spirit of the invention, the scope of which is defined in theappended claims.

What is claimed is:
 1. A motor control device comprising: first andsecond pairs of transistors, each transistor being connected to avoltage source, the first pair of transistors being adapted forconnection to terminals of the motor and operative when both transistorsof the first pair are conducting to connect the motor to the voltagesource such that the motor is driven in a forward direction, the secondpair of transistors being adapted for connection to the terminals of themotor and operative when both transistors of the second pair areconducting to reversibly connect the motor to the voltage source suchthat the motor is driven in a reverse direction;first selective meansfor rendering the first pair or the second pair of transistorsconductive alternately to drive the motor in the forward direction orthe reverse direction; and second selective means for rendering onetransistor of the first pair and one transistor of the second pairconductive to form a closed loop which short-circuits the motor andcauses dynamic braking of the motor.
 2. A device according to claim 1,wherein said transistors each have an emitter and a collector, and eachpair of said transistors comprises complementary connected PNP and NPNtransistors, all of said transistors being operated in a groundedemitter configuration and being adapted to have their collectorsconnected to the motor.
 3. A device according to claim 2, wherein thecurrent flow through the transistors of each pair is in a forwarddirection when the motor is driven in the forward and in the reversedirections, whereas the current flow through one of the transistorsforming the closed loop which short-circuits the motor is in the reversedirection during dynamic braking.
 4. A device according to claim 1,wherein said first selective means comprises a first semiconductordriver for simultaneously rendering conductive the transistors of thefirst pair when a first control signal is applied to a control terminalof the first driver to render the first driver conductive, and a secondsemiconductor driver for simultaneously rendering conductive thetransistors of the second pair when a second control signal is appliedto a control terminal of the second driver to render the second driverconductive, and wherein the second selective means comprises a thirdsemiconductor driver for rendering conductive said one transistor of thefirst pair and said one transistor of the second pair to form saidclosed loop and short-circuit the motor, when a third control signal isapplied to a control terminal of the third driver.
 5. A device accordingto claim 4 further comprising a control circuit for generating saidcontrol signals, the control circuit having first, second, and thirdterminals connected to the control terminals of the first, second, andthird drivers, respectively.
 6. A device according to claim 5, whereinthe control circuit is a digital logic circuit having means forpreventing the generation of more than one of said control signals at atime.
 7. A device according to claim 6, wherein the preventing meansincludes gate means for ensuring the control signals on the first andsecond terminals are of opposite polarities, except when the motor isstopped or is being dynamically braked.
 8. A device according to claim6, wherein the control circuit includes a timer circuit for generatingthe third control signal on the third terminal have a predetermined timeduration.
 9. A device according to claim 1, wherein the first selectivemeans and the second selective means comprise a digital logic controlcircuit having a plurality of output terminals, each output terminalbeing connected to a control terminal of a corresponding one of saidtransistors, the control circuit having means for generating controlsignals on said output terminals for selectively rendering thetransistors conductive.
 10. A device according to claim 9 wherein thecontrol circuit includes a timer circuit for generating a control signalhaving a predetermined time duration, such control signal being employedfor controlling the transistors of each pair which form the closed loopfor short-circuiting the motor, thereby causing dynamic braking of themotor for said predetermined time duration.
 11. A device according toclaim 5 or 9, wherein the control circuit receives input signalsrepresentative of the condition of a mechanism being driven by themotor, and the control circuit generates said control signals inresponse to said input signals.
 12. A motor control device comprising:abridge circuit comprising first, second, third and fourth arms, each armincluding a transistor having a collector and an emitter, the first andthird arms being connected together and to one terminal of a voltagesource and forming a first pair of adjacent arms, the second and fourtharms being connected together and to an opposite polarity terminal ofthe voltage source and forming another pair of adjacent arms, the firstand fourth arms being connected together and to a first terminal of themotor and the second and third arms being connected together and toanother terminal of the motor, the first and second arms and the thirdand fourth arms respectively constituting first and second pairs ofopposing diagonal arms, the transistors in the first pair of adjacentarms being PNP type transistors, and the transistors in said other pairof adjacent arms being NPN type transistors, each transistor beingoperated in a grounded emitter configuration; first means for renderingconductive the transistors in the first pair of opposing diagonal armsof the bridge circuit to cause the motor to be driven in a forwarddirection and for alternately rendering conductive the transistors inthe second pair of opposing diagonal arms of the bridge circuit to causethe motor to be driven in a reverse direction, and second means forrendering conductive the transistors in one pair of said adjacent armsto form a closed loop which short-circuits the motor and causes dynamicbraking of the motor.
 13. A device according to claim 12, wherein saidfirst means comprises a first semiconductor driver for renderingconductive the transistors in the first pair of opposing diagonal armsto drive the motor in the forward direction upon a first control signalbeing applied to a control terminal of the first driver to render thefirst driver conductive, and a second semiconductor driver for renderingconductive the transistors in the second pair of opposing diagonal armsto drive the motor in the reverse direction upon a second control signalbeing applied to a control terminal of the second driver to render thesecond driver conductive, and wherein said second means comprises athird semiconductor driver for rendering conductive the transistors inone of the pairs of adjacent arms to form said closed loop andshort-circuit the motor upon a third control signal being applied to acontrol terminal of the third driver.
 14. A device according to claim 13further comprising a control circuit having first, second, and thirdoutput terminals respectively connected to the control terminals of thefirst, second, and third drivers, the control circuit being responsiveto an input signal thereto and having means for generating said controlsignals such that only one of said first, second and third controlsignals is generated at a time.
 15. A device according to claim 14,wherein the control circuit includes a timer circuit for generating saidthird control signal on the third output terminal to have apredetermined time duration that is sufficient to ensure that the motoris completely stopped.